1. Field of the Invention
The invention relates in general to a liquid crystal display and a driving method thereof, and more particularly to a liquid crystal display of improving a frame flicker phenomenon, and a driving method thereof.
2. Description of the Related Art
In order to make the liquid crystal display possess a better image quality, a backlight module for controlling on and off of a lighting device to reduce the frame retained image and enhance the motion picture quality has been disclosed.
The dimming methods for the backlight module may be divided into an analog dimming method and a burst dimming method.
FIG. 1 shows a waveform of an output voltage for changing the luminance of a lamp by way of analog dimming. The analog dimming utilizes a control inverter to drive the amplitude Vp of the output voltage of the lamp to change the luminance of the lamp. When the amplitude Vp of the output voltage becomes larger, the luminance of the impulse type backlight module increases. Inversely, when the amplitude Vp of the output voltage becomes smaller, the luminance of the impulse type backlight module decreases.
Because the current liquid crystal display is developed toward the trend of large-scale specification, the uniformity of the impulse type backlight module deteriorates due to the leakage current if the impulse type backlight module adopts the analog dimming method. Thus, the analog dimming only can reach the 70% to 100% of the luminance dimming range.
FIG. 2 shows a waveform of a dimming control signal for changing the luminance of a lamp by way of burst dimming. In order to enlarge the range of luminance dimming, most of the current impulse type backlight modules utilize the burst dimming method. The burst dimming is also referred to as the digital dimming or the pulse width modulation dimming (PWM Dimming), in which the luminance of the lamp is changed as a duty cycle TDuty of the dimming control signal is changed. When the duty cycle TDuty of the dimming control signal becomes larger, the luminance of the impulse type backlight module increases. Inversely, when the duty cycle TDuty of the dimming control signal becomes smaller, the luminance of the impulse type backlight module decreases. Compared to the analog dimming method, the range of the burst dimming method can reach 30% to 100% of luminance.
However, the frame luminance sensed by the human eyes depends on the dimming frequency of the flicker type backlight module and the frame rate of the liquid crystal display panel. When the dimming frequency interacts with the frame rate to form a new flicker frequency falling within the range that can be sensed by the human eyes, the frame flicker phenomenon is formed on the liquid crystal display panel, and the human eyes may feel uncomfortable.
FIG. 3 shows a waveform measured in a conventional liquid crystal display using the burst dimming. For example, when the dimming frequency (i.e., the backlight frequency) is 208 Hz and the frame rate is 60 Hz, a serious frame flicker phenomenon is formed at 31 to 36 Hz of FIG. 3.
FIG. 4 is a schematic illustration showing a conventional liquid crystal display. In addition, because a scan driver 460 of a conventional liquid crystal display 40 sequentially outputs scan signals Sscan(1) to Sscan(m) to drive each row of pixels on a liquid crystal display panel 410, as shown in FIG. 4, the orientations of the liquid crystal molecules of the pixels in different display areas are influenced due to the different timings of the scan signals. Thus, the pixels in different display areas do not reach the maximum transmission at the same time, and the liquid crystal response curves in different display areas differ from each other by a liquid crystal delay phase.
FIG. 5 shows liquid crystal response curves of a liquid crystal display in different horizontal display areas. Row “a” of pixels, row “b” of pixels, row “c” of pixels and row “d” of pixels on the liquid crystal display panel 410 are located in different horizontal display areas, and the transmissions of row “a” of pixels, row “b” of pixels, row “c” of pixels and row “d” of pixels form liquid crystal response curves LC(a) to LC(d) with the time. The liquid crystal delay phase exists between adjacent two of the liquid crystal response curves LC(a) to LC(d), and the liquid crystal delay phase substantially equals the delay time td. For example, row “a” of pixels in the liquid crystal response curve LC(a) reaches the maximum transmission at time t1, and row “b” of pixels in the liquid crystal response curve LC(b) reaches the maximum transmission at time t2. The liquid crystal delay phase between the liquid crystal response curve LC(a) and the liquid crystal response curve LC(b) equals the delay time td, and td=t2−t1.
Because the timings of the scan signals Sscan(1) to Sscan(m) are different from one another, the liquid crystal delay phases between adjacent two of the liquid crystal response curves LC(a) to LC(d) tend to make the human eyes feel that the color temperature of the frame of the liquid crystal display panel 410 is not uniform and the phenomenon of slow movement of the horizontal black band is thus formed.